1. Field of the Invention
The present invention relates to an electroluminescence (EL) polymer and an EL element having the same, and more particularly, to spirobifluorene compounds which can provide excellent processibility with improved solubility in organic solvents, an EL polymer obtained therefrom and an EL element having the same.
2. Description of the Related Art
In general, an electroluminescence (EL) element using polymer materials is formed by sequentially depositing a transparent electrode, an electrically conducting buffer layer, a light-emitting polymer layer, a dielectric layer and a metal electrode on a substrate. Here, if the transparent electrode and the metal electrode are connected to positive (+) and negative (−) terminals of power, holes are supplied to the light-emitting polymer layer through the transparent electrode and electrons are supplied to the light-emitting polymer layer through the metal electrode. The holes and electrons supplied to the light-emitting polymer layer are combined therein to create exciton. Upon deactivation of the excitation to a ground state, light corresponding to a band gap of the light-emitting polymer is emitted, thereby achieving luminescence. The color of the emitted light can be varied according to the band gap of the light-emitting polymer.
Since organic EL elements are quick in response and self-illuminating, they require no additional back light source. Also, the organic EL elements have excellent luminance and are not dependent upon viewing angle of display. Types of the organic EL elements include low molecular weight EL elements and polymer EL elements according to materials used. The polymer EL elements can be fabricated through solution process while the low molecular weight EL elements are usually prepared through vacuum deposition process, those have excellent processability and their operating voltage is low, that is, power consumption is small.
Since Cambridge group first found and disclosed in 1990 that light is emitted from poly(1,4-phenylenevinylene), which is a conjugated double bond polymer, as a voltage is applied, the field of EL polymer has been greatly advanced in terms of materials and devices. Covion Organic Semiconductors GmbH, based in Germany, developed poly(1,4-phenylenevinylene) based polymer materials soluble in organic solvents and having good chemical stability and light-emitting efficiency. The Dow Chemical Company (USA) developed poly(fluorene) based polymer materials having better chemical stability and light-emitting efficiency than poly(1,4-phenylenevinylene) based polymer materials.
Poly(phenylene)-, specifically, poly(fluorene)-based materials are known as promising blue light-emitting materials. Woo, Inbasekaran, Shiang and Roof developed EL polymers using a polymerization technique of polymers, called Suzuki coupling, and disclosed poly(fluorene) based EL materials in WO 97/05184 in 1997. In 1999, poly(fluorenes) with polar side chains were proposed by Pei, Yu and Yang, but the polar side chains promote formation of excimer, which is one of problems with poly(fluorene), resulting in degradation in color purity (see U.S. Pat. No. 5,900,327 and J. Appl. Phys., Vol. 81, pp 3294-3298). Use of the proposed polymers allowed manufacture of light-emitting electrochemical cells using polar side chains, with a very high efficiency of light emission. In order to solve the problem of degradation in color purity of blue electoluminescence due to formation of excimer by conventional poly(fluorenes), Chen, Klaerner, Miller and Scott of IBM Corporation developed poly(anthracene-co-fluorene) copolymers obtained by copolymerizing the poly(fluorenes) with anthracenes (see U.S. Pat. No. 5,998,045). It is quite important for blue light-emitting polymers to exhibit high efficiency of light emission while maintaining color purity.
In order to improve color purity of poly(fluorenes), Kreuder, Lupo, Salbeck, Schenk and Stehlin disclosed poly(spirobifluorenes) having a spiro framework and copolymers thereof (see U.S. Pat. No. 5,621,131). Also, they disclosed polymer EL materials using new spiro polymers having a Spiro framework other than spirobifluorene or having a hetero element substituted (see U.S. Pat. Nos. 5,763,636 and 5,859,211). Although these materials have high color purity and color stability, they are poor in view of processibility, that is, poor polymer film forming properties due to low solubility in organic solvents.